To apply for SSC in the Flying Branch, you can apply through AFCAT. This is for 14 years with no further extension. The eligibility criteria says that:

• You should be aged between 20-24 at the time of commencement of the course
• You should be an Indian National
• Your marital status should be single
• You should have completed your Bachelor's degree in any discipline.
• You should have studied Mathematics and Physics at 10+2 level and should have score a minimum of 60% of marks in both these subjects separately.
• If it is B.Tech, it should be a 4 year course and should have completed your BE/B.Tech with a minimum aggregate of 60%.

This information is available at: https://afcat.cdac.in/AFCAT/CareerAsPerBranches.html

Now regarding B.Tech Lateral Entry, it says that B.Tech should have been 4 years but you have studied only 3 years due to Lateral entry. Please mail them at: afcatcell@gmail.com for a clarification on this. Going by what has been mentioned in the official website, lateral Entry is allowed for Integrated courses but not regular courses.